Three dimensional photonic imaging systems, also referred to as three-dimensional (3D) cameras, are capable of providing distance measurements and photonic measurements for physical objects in a scene. Applications for such 3D cameras are industrial inspection, selective robot vision, 3D modeling, geographic surveying, and forensic analysis.
3D cameras can be implemented with a variety of technologies, with each combination of technologies presenting certain limitations that render the cameras ineffective in broad-use applications. Stereo vision 3D cameras implement two or more imaging arrays in a fixed, highly-calibrated configuration and utilize triangulation of common points within the fields of view to establish distances to each of the common points. Stereo vision systems suffer from image distance inaccuracies due to occlusion and parallax. Furthermore, distance accuracies suffer when the baseline distance between image arrays is small relative to the distances being measured. Lastly, stereo 3D cameras are expensive due to the need for multiple image arrays and the requirement for high precision for the baseline offset between the image arrays.
Time-of-flight (TOF) systems utilize light sources, such as lasers, that are pulsed or modulated so they provide pulses of light for illuminating scenes in conjunction with an imaging system for measuring the amplitude and timing of the light reflected from the objects in the scene. Distances to points in the scene are determined using the known speed of light for all of the reflected signals. The imaging systems for TOF devices comprise a camera with a photodetector array, typically fabricated using CCD or CMOS technology, and a method for rapidly gating the collection times for the photodetector elements. Reflected light is captured by the photodetector elements during the specific gating cycles.
Some TOF systems only utilize the timing between light pulses and gated photodetectors to determine 3D object distances. Other TOF systems utilize the amount of received light during a gated capture cycle to establish object distances. The accuracy of these systems depends on the uniformity of incident light and the speed of the gating mechanism for the photodetectors.
Utilizing gated photodetectors is an effective method to establish distances to objects in a scene. By precisely controlling the timing between incident light pulses and gated photodetectors the distances to objects in certain distance bands can be accurately determined. For establishing object distances for other distance bands, subsequent light and gated photodetector cycles are utilized while the stationary objects and stationary camera are maintained in their present configurations and orientations. Any movement of the camera and/or objects in the scene will result in distance measurement bands that are not registered with one another.
A 3D camera described in U.S. Pat. No. 4,935,616 utilizes a modulated source and imaging system. A preferred embodiment of this system uses a CW laser and utilizes the phase difference between the incident and reflected signals to establish the distances to objects.
Another 3D camera is described in U.S. Pat. No. 5,081,530. This system utilizes a pair of gates for each photodetector element. Distances to objects are determined from the ratio of differences between the sampled energy at the two gated elements.
U.S. Pat. Nos. 7,362,419 and 7,755,743 each utilize modulated light intensity sources and phase ranges to detect phase shifts between emitted and detected signals. An embodiment of U.S. Pat. No. 8,159,598 utilizes modulated light intensity and phase shift detection for time of flight determination. Other embodiments of U.S. Pat. No. 8,159,598 utilize a high resolution color path with a low resolution distance path to determine 3D information for a detector or a group of detectors.
U.S. Pat. No. 8,102,426 to Yahav describes 3D vision on a chip and utilizes an array of photodetector elements that are gated at operative times to establish object distances in a scene. Photodetector sites are utilized for either TOF distance measurement or for the determination of object color. Embodiments of Yahav describe utilizing groups or bands of photodetector elements to establish the various distance bands. Other embodiments of Yahav describe a single distance band for each capture cycle, with full scene distances established utilizing sequences of capture cycles. Although not specified in Yahav, the requirement for the embodiments is no movement of the camera and the objects in the scene throughout the sequence of capture cycles.
For real-world applications like autonomous vehicle navigation, mobile mapping, agriculture, mining, and surveillance it is not practical to require little or no relative movement between a 3D camera and objects in a scene during a sequence of imaging cycles. Furthermore, most of the real-world situations occur in scenes that have widely-varying ambient light conditions. It is desirable to have a 3D camera that captures images at a sufficient rate to accurately measure objects in transient scenes where the objects, camera or both are in motion. Furthermore, it is desirable to have an active-light-source 3D camera that produces accurate distances and colors for daytime and nighttime imaging. Furthermore, it is desirable to have a camera that can “see through” atmospheric conditions that dramatically limit the visibility of two-dimensional cameras.